Controlling atomizer surfaces by varying ascorbic acid pyrolysis conditions to modify activation energies for gold desorption in electrothermal atomic absorption spectrometry

Abstract

As a chemical modifier on kinetic parameters for atomic absorption signals for gold, the effect of the temperature and hold time of the ascorbic acid pyrolysis step has been investigated. The signal for gold, dispersed individually into micro-sized pores, was analysed using Arrhenius plots based on solid-phase kinetics. The first kinetic order, corresponding to desorption of individually dispersed atoms, was chosen as best possible fit for all plots. Using mass dependency of the absorbance–time profile characteristics and activation energy supported the first kinetic order. Increasing the temperature of pyrolysis of ascorbic acid above 1676 K decreased the activation energy (Ea) for desorption of the gold atoms from 325 ± 11 kJ mol−1 to 108 ± 10 kJ mol−1. Below a pyrolysis temperature of 1676 K, the Ea value was nearly constant. The high Ea value corresponded to desorption of the adatoms dispersed at active sites on a rough surface with a surface fractal dimension (D) of 2.80 ± 0.16, evaluating quantitatively the degree of surface roughness. The low Ea value corresponded to D = 2.04 ± 0.16. If D is 2, the surface is completely flat. The closer the D value is to 3, the rougher the surface. At pyrolysis temperatures of 1003 and 1470 K, the Ea decreased from 309 ± 13 and 317 ± 11 kJ mol−1 and reached an average value of 283 ± 8 and 260 ± 18 kJ mol−1, respectively, by increasing the hold time of pyrolysis of ascorbic acid. At 1720 K, Ea decreased from 258 ± 6 kJ mol−1 and reached an average value of 159 ± 8 kJ mol−1 with increasing hold time of pyrolysis. The change in Ea, corresponding to pyrolysis temperature, is discussed from the viewpoint of a change in the degree of interaction with the pore wall surface relating to a change in degree of the surface roughness.